<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    let grade = +prompt('请输入您的成绩')
    if (grade >= 90) {
      alert('A')

    } else if (grade >= 80) {
      alert('B')

    } else if (grade >= 70) {
      alert('C')

    } else if (grade >= 60) {
      alert('D')

    } else {
      alert('E')
    }
  </script>
</body>

</html>